Constant force spring with active bias

ABSTRACT

A compensated constant force spring device includes a bracket, a drum rotatably supported by the bracket, and a constant force spring wound on the drum. A motor is fixed to the bracket and provides a compensating force to the drum. The motor may be located in an interior volume of the drum. A control module may coupled to the motor to control the compensating force. A position sensor may be coupled to the control module. The compensating force may be responsive to a signal from the position sensor. The constant force spring may support a load and counterbalance gravitational forces on the load. The compensating force may be adjusted when the load approaches an end of a range of travel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority to U.S. ProvisionalApplication No. 61/954,452 filed Mar. 17, 2014, entitled “CONSTANT FORCESPRING WITH ACTIVE BIAS”, and U.S. Provisional Application No.62/019,311 filed Jun. 30, 2014, entitled “CONSTANT FORCE SPRING WITHACTIVE BIAS”, each of which are incorporated herein by reference intheir entirety and for all purposes.

FIELD

Embodiments of the invention relate to the field of field of constantforce springs; and more specifically, to constant force springs with anactive bias for supporting surgical instruments at an adjustable height.

BACKGROUND

Minimally invasive medical techniques have been used to reduce theamount of extraneous tissue which may be damaged during diagnostic orsurgical procedures, thereby reducing patient recovery time, discomfort,and deleterious side effects. Traditional forms of minimally invasivesurgery include endoscopy. One of the more common forms of endoscopy islaparoscopy, which is minimally invasive inspection or surgery withinthe abdominal cavity. In traditional laparoscopic surgery, a patient'sabdominal cavity is insufflated with gas and cannula sleeves are passedthrough small (approximately 1¼ cm.) incisions in the musculature of thepatient's abdomen to provide entry ports through which laparoscopicsurgical instruments can be passed in a sealed fashion.

The laparoscopic surgical instruments generally include a laparoscopefor viewing the surgical field and working tools defining end effectors.Typical surgical end effectors include clamps, graspers, scissors,staplers, and needle holders, for example. The working tools are similarto those used in conventional (open) surgery, except that the workingend or end effector of each tool is separated from its handle by anapproximately 30 cm. long extension tube, for example, so as to permitthe operator to introduce the end effector to the surgical site and tocontrol movement of the end effector relative to the surgical site fromoutside a patient's body.

In order to provide improved control of the working tools, it may bedesirable to control the instrument with teleoperated actuators. Thesurgeon may operate controls on a console to indirectly manipulate theinstrument that is connected to the teleoperated actuators. Theinstrument is detachably coupled to the teleoperated actuators so thatthe instrument can be separately sterilized and selected for use asneeded instrument for the surgical procedure to be performed. Theinstrument may be changed during the course of a surgery.

Performing surgery with teleoperated actuated instruments creates newchallenges. One such challenge is providing a teleoperated motormechanism that supports the teleoperated actuated surgical instrumentsthat can be positioned with respect to the patient. These mechanisms arefairly heavy, weighing perhaps twelve to twenty-four kilograms. Themechanisms must be moved over a patient and positioned carefully.Therefore it is necessary to counterbalance the surgical instrument, therelated actuators, and the support structure so that the surgicalinstrument can safely and easily be positioned. Counterbalancing is mademore difficult because the various surgical instruments may have variousweights.

It would be desirable to provide a way of counterbalancing a surgicalinstrument, its related actuators, and support structure that iseffective with changeable surgical instruments of various weights.

SUMMARY

A compensated constant force spring device includes a bracket, a drumrotatably supported by the bracket, and a constant force spring wound onthe drum. A motor is fixed to the bracket and provides a compensatingforce to the drum. The motor may be located in an interior volume of thedrum. A control module may coupled to the motor to control thecompensating force. A position sensor may be coupled to the controlmodule. The compensating force may be responsive to a signal from theposition sensor. The constant force spring may support a load andcounterbalance gravitational forces on the load. The compensating forcemay be adjusted when the load approaches an end of a range of travel.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention by way of example and not limitation. Inthe drawings, in which like reference numerals indicate similarelements:

FIG. 1 is a view of an illustrative patient-side portion 100 of ateleoperated surgical system.

FIG. 2 is a side view of a surgical instrument for use with ateleoperated actuator.

FIG. 3 is a side view of setup joint for a surgical instrument.

FIG. 4 is a side view of the setup joint shown in FIG. 3 with a housingremoved.

FIG. 5 is a perspective view of a portion of the setup joint shown inFIG. 4.

FIG. 6 is a perspective view of a constant force spring assembly.

FIG. 7 is an exploded perspective view of a portion of the constantforce spring assembly shown in FIG. 6.

FIG. 8 is a cross-section view of a portion of the constant force springassembly taken along line 8-8 in FIG. 6.

FIG. 9 is a perspective view of a constant force spring assembly withadditional components of the setup joint.

DESCRIPTION OF EMBODIMENTS

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures, and techniques have not been shown in detail inorder not to obscure the understanding of this description.

In the following description, reference is made to the accompanyingdrawings, which illustrate several embodiments of the present invention.It is understood that other embodiments may be utilized, and mechanicalcompositional, structural, electrical, and operational changes may bemade without departing from the spirit and scope of the presentdisclosure. The following detailed description is not to be taken in alimiting sense, and the scope of the embodiments of the presentinvention is defined only by the claims of the issued patent.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like may be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the figures. It will be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(e.g., rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein interpreted accordingly.

As used herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising” specify the presence of stated features, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, steps, operations,elements, components, and/or groups thereof.

FIG. 1 is a view of an illustrative patient-side portion 100 of ateleoperated surgical system, in accordance with embodiments of thepresent invention. The patient-side portion 100 includes supportassemblies 110 and one or more surgical instrument manipulators 112 atthe end of each support assembly. The support assemblies optionallyinclude one or more unpowered, lockable setup joints that are used toposition the surgical instrument manipulator(s) 112 with reference tothe patient for surgery. As depicted, the patient-side portion 100 restson the floor. In other embodiments the patient-side portion may bemounted to a wall, to the ceiling, to the operating table 126, whichalso supports the patient's body 122, or to other operating roomequipment. Further, while the patient-side portion 100 is shown asincluding four manipulators 112, more or fewer manipulators 112 may beused. Still further, the patient-side portion 100 may consist of asingle assembly as shown, or it may include two or more separateassemblies, each optionally mounted in various possible ways.

Each surgical instrument manipulator 112 supports one or more surgicalinstruments 120 that operate at a surgical site within the patient'sbody 122. Each manipulator 112 may be provided in a variety of formsthat allow the associated surgical instrument to move with one or moremechanical degrees of freedom (e.g., all six Cartesian degrees offreedom, five or fewer Cartesian degrees of freedom, etc.). Typically,mechanical or control constraints restrict each manipulator 112 to moveits associated surgical instrument around a center of motion on theinstrument that stays stationary with reference to the patient, and thiscenter of motion is typically located to be at the position where theinstrument enters the body.

The term “surgical instrument” is used herein to describe a medicaldevice configured to be inserted into a patient's body and used to carryout surgical or diagnostic procedures. The surgical instrument typicallyincludes an end effector associated with one or more surgical tasks,such as a forceps, a needle driver, a shears, a bipolar cauterizer, atissue stabilizer or retractor, a clip applier, an anastomosis device,an imaging device (e.g., an endoscope or ultrasound probe), and thelike. Some surgical instruments used with embodiments of the inventionfurther provide an articulated support (sometimes referred to as a“wrist”) for the end effector so that the position and orientation ofthe end effector can be manipulated with one or more mechanical degreesof freedom in relation to the instrument's shaft. Further, many surgicalend effectors include a functional mechanical degree of freedom, such asjaws that open or close, or a knife that translates along a path.Surgical instruments may also contain stored (e.g., on a semiconductormemory inside the instrument) information that may be permanent or maybe updatable by the surgical system. Accordingly, the system may providefor either one-way or two-way information communication between theinstrument and one or more system components.

A functional teleoperated surgical system will generally include avision system portion (not shown) that enables the operator to view thesurgical site from outside the patient's body 122. The vision systemtypically includes a surgical instrument that has a video-image-capturefunction 128 (a “camera instrument”) and one or more video displays fordisplaying the captured images. In some surgical system configurations,the camera instrument 128 includes optics that transfer the images fromthe distal end of the camera instrument 128 to one or more imagingsensors (e.g., CCD or CMOS sensors) outside of the patient's body 122.Alternatively, the imaging sensor(s) may be positioned at the distal endof the camera instrument 128, and the signals produced by the sensor(s)may be transmitted along a lead or wirelessly for processing and displayon the video display. An illustrative video display is the stereoscopicdisplay on the surgeon's console in surgical systems commercialized byIntuitive Surgical, Inc., Sunnyvale, Calif.

A functional teleoperated surgical system will further include a controlsystem portion (not shown) for controlling the movement of the surgicalinstruments 120 while the instruments are inside the patient. Thecontrol system portion may be at a single location in the surgicalsystem, or it may be distributed at two or more locations in the system(e.g., control system portion components may be in the system'spatient-side portion 100, in a dedicated system control console, or in aseparate equipment rack). The teleoperated master/slave control may bedone in a variety of ways, depending on the degree of control desired,the size of the surgical assembly being controlled, and other factors.In some embodiments, the control system portion includes one or moremanually-operated input devices, such as a joystick, exoskeletal glove,a powered and gravity-compensated manipulator, or the like. These inputdevices control teleoperated motors which, in turn, control the movementof the surgical instrument.

The forces generated by the teleoperated motors are transferred viadrivetrain mechanisms, which transmit the forces from the teleoperatedmotors to the surgical instrument 120. In some telesurgical embodiments,the input devices that control the manipulator(s) may be provided at alocation remote from the patient, either inside or outside the room inwhich the patient is placed. The input signals from the input devicesare then transmitted to the control system portion. Persons familiarwith telemanipulative, teleoperative, and telepresence surgery will knowof such systems and their components, such as the da Vinci® SurgicalSystem commercialized by Intuitive Surgical, Inc. and the Zeus® SurgicalSystem originally manufactured by Computer Motion, Inc., and variousillustrative components of such systems.

As shown, both the surgical instrument 120 and an optional entry guide124 (e.g., a cannula in the patient's abdomen) are removably coupled tothe distal end of a manipulator 112, with the surgical instrument 120inserted through the entry guide 124. Teleoperated actuators in themanipulator 112 move the surgical instrument 112 as a whole. Themanipulator 112 further includes an instrument carriage 130. Thesurgical instrument 120 is detachably connected to the carriage 130. Theteleoperated actuators housed in the carriage 130 provide a number ofcontroller motions which the surgical instrument 120 translates into avariety of movements of the end effector on the surgical instrument.Thus the teleoperated actuators in the carriage 130 move only one ormore components of the surgical instrument 120 rather than theinstrument as a whole. Inputs to control either the instrument as awhole or the instrument's components are such that the input provided bya surgeon to the control system portion (a “master” command) istranslated into a corresponding action by the surgical instrument (a“slave” response).

FIG. 2 is a side view of an illustrative embodiment of the surgicalinstrument 120, comprising a distal portion 250 and a proximal controlmechanism 240 coupled by an elongate tube 210. The distal portion 250 ofthe surgical instrument 120 may provide any of a variety of endeffectors such as the forceps 254 shown, a needle driver, a cauterydevice, a cutting tool, an imaging device (e.g., an endoscope orultrasound probe), or a combined device that includes a combination oftwo or more various tools and imaging devices. In the embodiment shown,the end effector 254 is coupled to the elongate tube 210 by a “wrist”252 that allows the orientation of the end effector to be manipulatedwith reference to the instrument tube 210.

Referring again to FIG. 1, a surgical instrument 120, its relatedactuators 130, and support structure may be supported vertically by anextensible support 110.

FIG. 3 is a side view of the extensible support 110. A vertical column300 hangs down from a housing. An arm 304 is supported by the lower end306 of the vertical column 300. The arm in turn supports the surgicalmanipulator and its associated teleoperated actuators. A surgicalinstrument may be coupled to the surgical manipulator and be supportedin turn by the arm 304 and the vertical column 300.

FIG. 4 is a side view of the extensible support 110 shown in FIG. 3 withthe cover 302 removed from the housing. The upper end 410 of thevertical column 300 is coupled to a sliding assembly, such as a track408 and carriage 406 assembly that allows the vertical column to move upand down to adjust the height of the surgical manipulator over thepatient.

FIG. 5 is a perspective view of the housing portion of the extensiblesupport shown in FIG. 4. Some components have been removed to allow thetrack 408 and carriage 406 assembly to be seen more clearly.

Referring again to FIG. 4, a constant force spring 400 is coupled to thevertical column 300 at a lower end 402 of the constant force spring. Theconstant force spring 400 is rolled around a drum 404 that is supportedby the upper end of the extensible support assembly 110. The constantforce spring counteracts the force of gravity acting on the verticalcolumn 300 and the structures it supports including the surgicalinstrument.

Constant force springs may be constructed as a rolled ribbon of springsteel such that the spring is relaxed at a lower stress state whenrolled up as opposed to being extended. As it is unrolled, the restoringforce comes primarily from the portion of the ribbon near the roll ofrelaxed spring. Specifically, the force comes from the region that isbeing transitioned from round to flat. No force comes from the portionthat is totally unrolled, or still rolled on the drum. Because thegeometry of that region remains nearly constant as the spring unrolls,the resulting force is nearly constant. A self-retracting steelmeasuring tape is an example of a constant force spring. While theconstant force spring 400 provides a nearly constant counterbalancingforce to support vertical column 300 and the attached structures, itwould desirable to provide a counterbalancing force that is moreconstant than what can be achieved with a constant force spring aloneand which can compensate for differing supported weights.

FIG. 6 is a perspective view of the constant force spring 400 shown inFIG. 4. A bracket 600, 602, 604 is supported by the upper end of theextensible support assembly 110. The bracket rotatably supports the drum404 around which the constant force spring 400 is rolled. The constantforce spring 400 may be fixed to the drum by the friction force createdbetween the surface of the drum and the constant force spring as itattempts to fully roll up to a relaxed diameter that is smaller than thedrum diameter.

FIG. 7 is an exploded view of the drum portion of the constant forcespring assembly shown in FIG. 5. A plate that includes an axial support700 is fixed to one side 604 of the bracket that supports the drum 404from the upper end of the extensible support assembly 110. The axialsupport 700 may provide a bearing that rotatably supports the drum 404.The constant force spring assembly includes a motor 710, which may be abrushless DC motor, having a stator 702 and a rotor 706 that provides anactive rotational force that turns the drum 404. The force provided bythe motor is translated into a linear force acting on the verticalcolumn 300. The torque provided by the motor is translated into a linearforce across the constant force spring acting on the vertical column300. The torque provided by the motor may add to or subtract from thecounterbalancing force provided by the constant force spring 400.

The motor includes a stator 702 that is fixed to a second side 602 ofthe bracket that supports the drum 404. A bearing 708 may be supportedby a portion 704 of the motor stator to provide a rotatable support forthe drum 404. The motor further includes a rotor 706 that is fixed tothe drum 404.

FIG. 8 is cross-section view of the drum portion of the constant forcespring assembly taken along section line 8-8 shown in FIG. 6. As can beseen in this view, the bracket 600, 602, 604 and the motor stator 702which are fixed together as one sub-assembly. The bracket and motorprovide a ground reference for the rotating drum 404 and motor rotor 706which are fixed together as a second sub-assembly.

A first bearing 800 that is supported by the axial support 700 on thebracket 604 supports a closed end of the drum 404. Providing a closedend to the drum 404 may increase the strength of the drum so that it cansupport the rolling force of the constant force spring 400 when rolledonto the drum prior to being assembled with the motor. A second bearing708 that is supported by a shoulder 704 on the motor stator 702 supportsan open end of the drum 404. Thus the drum 404 and motor rotor 706 aresupported by bearings 708, 800 that are in turn supported by thegrounded bracket 600,602, 604 and motor stator 702. Thus the motor 710is located in an interior volume 712 of the drum 404. In otherembodiments, other arrangements may be used to rotatably support thedrum and motor rotor with respect to the bracket and motor stator.

In other embodiments, the motor may be provided in locations other thanthe interior volume of the drum that supports the constant force spring.For example, the rotor of the motor may be extended by a shaft that isdirectly coupled to the coaxial drum. Alternatively, the drum and themotor may not be coaxial and the rotor of the motor may be coupled tothe drum by a mechanical transmission such as a belt, gears, and/or achain and sprocket drive. It is also possible to fix what has beenidentified as the motor rotor to the bracket and couple the drum to whathas been identified as the motor stator. In this configuration the outerpart of the motor and the coupled drum rotate around the inner part ofthe motor.

It will be appreciated that the constant force spring could be replacedby a flat belt and the motor could provide the force to counterbalancethe force of gravity acting on the vertical column 300 and thestructures it supports including the surgical instrument. However, thiswould require a sizeable motor and a substantial amount of electriccurrent to support mechanisms that may weigh perhaps twelve totwenty-four kilograms. By providing a constant force spring as thecoupling between the motorized drum and the load, the constant forcespring provides the majority of the force required to support the load.The motor provides a biasing force that corrects for the variability ofthe load and the irregularities in the force provided by the constantforce spring.

In one embodiment, the constant force spring is sized to provideslightly more than the force required to support the heaviest load. Thusthe constant force spring will always lift the load. The motor is usedto provide a controllable downward force that acts against the forceprovided by the constant force spring to provide a “neutral buoyancy”for the vertical column and the structures it supports.

FIG. 9 is a perspective view of the constant force spring 400 shown inFIG. 6 with additional components shown. As previously discussed, thevertical column may be supported by a track 408 and carriage 406assembly. A brake may be provided to hold the vertical column in a fixedposition so that no power is required when the position of the verticalcolumn is not being changed. The brake may be in the form of a brakethat clamps the extended portion of the constant force spring in a fixedposition, a brake that prevents the drum from rotating, or a brake thatprevents the vertical column from moving, such as the magnetic brakeillustrated that magnetically grips an armature 904 with a magneticbrake shoe 906.

The motor includes a primary sensor 912, which is a rotary sensor thatprovides an absolute rotary position for the motor. One part of theprimary sensor is mounted on the motor rotor. The other part of theprimary sensor is mounted to a mechanical ground, such as the motorstator. The primary sensor is coupled to a control module 914 thatprovides controlled electrical current to the motor to provide thedesired motion and torque from the motor. The primary sensor is used formotor commutation and shaft speed control.

A secondary sensor 900, 902 may be provided to provide data for theposition of the vertical column to the control module 914. The secondarysensor 900, 902 may be mounted to the carriage that supports thevertical column and to a mechanical ground such as the frame thatsupports the stationary rails 408 on which the carriage slides. Thesecondary sensor 900, 902 provides an absolute linear position.

The secondary sensor 900, 902 provides a backup to the primary sensor912. Readings from the two sensors may be compared to confirm that thevertical column as sensed by the secondary sensor 900, 902 is moving asexpected based on the rotation of the motor as sensed by the primarysensor 912.

The secondary absolute position sensor 900, 902 may be used toperiodically calibrate the force required to compensate forirregularities in the force provided by the constant force spring 400and for fatigue of the spring resulting from extended use. Thecalibration routine uses the secondary sensor 900, 902 in conjunctionwith the motor and the primary motor sensor 912. Because the effectiveradius of the constant force spring 400 is a function of how much ispayed out, the ratio between the primary and secondary sensors isvariable. This variable ratio is taken into consideration in thecalibration. Such calibration will generally be needed infrequently.

Instruments supported by the vertical column may be provided withmachine readable identification that enables the control module 914 todetermine the amount of weight added to the vertical column by theinstrument. The machine readable identification may provide a generalweight for the type of instrument or a specific weight for theindividual instrument, either directly or by reference to a database ofinstrument information. The control module 914 is able to adjust theelectrical current provided to the motor to provide the desired forcefrom the motor to compensate for the weight of the installed surgicalinstrument.

The track 408 and carriage 406 assembly includes mechanical stops toprevent the carriages from running off the tracks. The mechanical stopsmay include rubber bumpers 908 that limit the carriage motion with onlya small amount of material deformation, perhaps 1 to 1.5 mm. Themechanical stops may also include spring stops 910 that limit thecarriage motion while providing a greater amount of yield, perhaps 3 to3.5 mm. It will be appreciated that even the spring stops 910 may stopthe carriage somewhat abruptly.

It may be desirable to provide some low compliance movement at the endsof the range of motion of the carriage 406 during operating table 126motion. If the carriage 406 is at the end of its range of motion,operating table 406 motion may not be possible. If the carriage 406reaches a mechanical stop 908, 910 during operating table 126 motion,the ability to move the operating table further is eliminated. Thespring plunger 910 may be allowed to push the carriage 406 off the limitof the mechanical stops 908, 910 to bring the carriage to rest at aposition that allows for low compliance movement of the carriage in bothdirections. It may be desirable to provide for mechanical brakes thathold the carriage in a fixed position. The brakes may be applied throughsoftware control that prevents brake application when the springplungers 910 are depressed. Therefore the software system lets thespring plungers 910 push the carriage 406 off the mechanical stops 908,910 before applying the brakes.

The control module 914 may use the motor to resist movement toward theend of the range of travel and bring the carriage to a stop over agreater distance to avoid abruptly stopping the movement of thecarriage. The control software implemented stop may gradually increasethe force required to move the carriage assembly over the end of therange of carriage travel, perhaps over the final 12 to 20 mm of carriagetravel. The increase in force provided by the motor to resist motionthrough the end of the range of travel for the carriage may be appliednon-linearly to stop the carriage with a desired deceleration profile.The software implemented stop may provide a force that graduallyincreases from a zero force at the beginning of the range, something maybe difficult to implement with a mechanical stop.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those of ordinary skill in the art. The description is thus tobe regarded as illustrative instead of limiting.

1.-14. (canceled)
 15. A compensated constant force spring devicecomprising: a bracket; a drum rotatably coupled to the bracket; aconstant force spring coupled to the drum; and a motor having a statorcoupled to the bracket and a rotor coupled to the drum.
 16. Thecompensated constant force spring device of claim 15 further comprisinga control module coupled to the motor.
 17. The compensated constantforce spring device of claim 16 further comprising a position sensorcoupled to the control module, wherein the control module causes themotor to provide a force responsive to a signal from the positionsensor.
 18. The compensated constant force spring device of claim 17further comprising a secondary position sensor coupled to the controlmodule, wherein the control module compares the signal from the positionsensor to a secondary signal from the secondary position sensor tocalibrate the force to be provided by the motor.
 19. The compensatedconstant force spring device of claim 15 further comprising a firstbearing supported by the bracket, wherein a first end of the drum isrotatably supported by the first bearing.
 20. The compensated constantforce spring device of claim 15 wherein the motor is located in aninterior volume of the drum.
 21. The compensated constant force springdevice of claim 20 further comprising a first bearing supported by thebracket and a second bearing supported by the stator, wherein a firstend of the drum is rotatably supported by the first bearing and a secondend of the drum is rotatably supported by the second bearing.
 22. Anextensible support device comprising: a bracket; a sliding assemblyfixed to the bracket; a vertical column coupled to the sliding assemblyat an upper end and supporting a load at a lower end, the slidingassembly allowing the vertical column to move in a vertical direction; adrum rotatably coupled to the bracket; a constant force spring having afirst end coupled to the drum and a second end coupled to the verticalcolumn, the constant force spring counterbalancing gravitational forceson the vertical column; and a motor having a stator coupled to thebracket and a rotor coupled to the drum.
 23. The extensible supportdevice of claim 22 further comprising a control module coupled to themotor.
 24. The extensible support device of claim 23 further comprisinga position sensor coupled to the control module, wherein the controlmodule causes the motor to provide a force responsive to a signal fromthe position sensor.
 25. The extensible support device of claim 24further comprising a secondary position sensor coupled to the controlmodule, wherein the control module compares the signal from the positionsensor to a secondary signal from the secondary position sensor tocalibrate the force to be provided by the motor.
 26. The extensiblesupport device of claim 24 wherein the control module further causes themotor to provide an additional force when the signal from the positionsensor indicates that the vertical column is approaching an end of arange of travel.
 27. The extensible support device of claim 22 furthercomprising a first bearing supported by the bracket, wherein a first endof the drum is rotatably supported by the first bearing.
 28. Theextensible support device of claim 22 wherein the motor is located in aninterior volume of the drum.
 29. The extensible support device of claim28 further comprising a first bearing supported by the bracket and asecond bearing supported by the stator, wherein a first end of the drumis rotatably supported by the first bearing and a second end of the drumis rotatably supported by the second bearing.
 30. An extensible supportdevice comprising: a bracket; a drum rotatably coupled to the bracket;means for supporting a load that is movable in a vertical direction; aconstant force spring coupled to the drum at a first end and coupled tothe means for supporting the load at a second end, the constant forcespring counterbalancing gravitational forces on the load; and means forapplying a controlled compensating force to the drum.
 31. The extensiblesupport device of claim 30 further comprising: first means for sensing aposition of the load; and means for adjusting the controlledcompensating force responsive to the position of the load.
 32. Theextensible support device of claim 31 further comprising: second meansfor sensing the position of the load; and means for to calibrating thecontrolled compensating force by comparing signals from the first andsecond means for sensing the position of the load.
 33. The extensiblesupport device of claim 31 further comprising means for furtheradjusting the controlled compensating force when the load is approachingan end of a range of travel.
 34. A method for supporting a load that ismovable in a vertical direction, the method comprising: supporting theload from a lower end of a vertical column; supporting an upper end ofthe vertical column on a sliding assembly fixed to a bracket; rotatablycoupling a drum to the bracket; coupling a first end of a constant forcespring to the drum; coupling a second end of the constant force springto the vertical column to counterbalance gravitational forces on thevertical column; and applying a controlled compensating force to thedrum.
 35. The method of claim 34 wherein the controlled compensatingforce is applied to the drum by a motor having a stator coupled to thebracket and a rotor coupled to the drum.
 36. The method of claim 34further comprising: sensing a position of the load with a positionsensor; and adjusting the controlled compensating force responsive tothe position of the load.
 37. The method of claim 36 further comprising:sensing the position of the load with a secondary position sensor; andcomparing the position of the load from the position sensor to theposition of the load from the secondary position sensor to calibrate thecontrolled compensating force to be provided.
 38. The method of claim 36further comprising adjusting the controlled compensating force when theload is approaching an end of a range of travel.